1. Field of the Invention
The invention relates to an ink composition suitably used for ink jet recording, an ink jet recording method, and a planographic printing plate obtained by using the ink composition, and a method for producing the same. More particularly, the invention relates to an ink composition that it is curable with high sensitivity by irradiation of activation radiation, to thus form a cured material having sufficient flexibility, and that is suitable for ink jet recording, an ink jet recording method, and a planographic printing plate obtained by using the ink composition and a method for producing the same.
2. Description of the Related Art
Examples of methods for forming an image on a recording medium such as paper based on an image data signal include electrophotographic methods, sublimation-type thermal transfer methods, fusion-type thermal transfer methods, and ink jet recording methods. The electrophotographic methods require a process of forming an electrostatic latent image on a photoreceptor drum by charging and light exposure, and the system therefore becomes complicated, resulting in increased production cost. The thermal transfer methods can be conducted by an inexpensive apparatus, but require use of ink ribbons, resulting in increased running cost and generation of waste.
The ink jet recording methods can also be conducted by an inexpensive apparatus. Moreover, an image is directly formed by ejecting ink only to the required area of an image recording medium, and the ink can therefore be efficiently used, leading to reduced running costs. Further, ink jet recording apparatuses are not noisy. Accordingly, ink jet recording methods are excellent image forming methods.
There is a demand for an ink composition, such as an ink composition for ink jet recording, which cures by irradiation of actinic radiation such as ultraviolet rays with high sensitivity to form high-quality images (radiation-curable ink composition). By achieving high sensitivity of an ink composition, the composition can be imparted with a high degree of curing by irradiation with actinic radiation, leading to reduced power consumption and reduced load applied to an actinic radiation generator, which lengthen the lifetime of the generator. Moreover, by sufficient curing, various advantages can be obtained, such as suppression of vaporization of uncured low-molecular weight substances and suppression of deterioration of strength of images formed. In addition, when the ink composition is used to form image regions of a planographic printing plate, improved strength of images, due to the high sensitivity, brings increased printing durability to the image regions.
Ink jet methods involving UV curing have been attracting attention recently, since the methods generate relatively weak odor and enable images to be recorded on recording media that do not have a quick drying property or an ink absorbing property. In such a method, benzil, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone, or 2-ethylanthraquinone is generally used as a photopolymerization initiator (see Chemical Review, Vol. 93, pp. 435-448, written by Bruce M. Monroe et al., and published in 1993). However, a photopolymerizable composition including such a photopolymerization initiator, which has low sensitivity, requires a long time for image-wise exposure when forming images. Therefore, in a case where the image to be formed has fine detail, if there is a slight vibration during the operation, images with good image quality cannot be obtained. Further, since a long exposure time is accompanied by an increase in energy radiation from the exposure light source, the effect of considerable heat generation resulting from radiation needs to be taken into account.
For the purpose of providing an ink composition that is highly safe, gives low skin irritation and low skin sensitizing, and allows printing of an image having high adhesiveness, even on a substrate (recording medium) on which it is usually difficult to directly form an image, with high sensitivity and with no ink bleeding by an ink jet recording method, compositions containing polymerizable compounds that are particular acrylate compounds have been proposed, (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2003-192943). However, since a polymer obtained by curing a polyfunctional acrylate used to improve sensitivity has a highly cross-linked three-dimensional structure, an image obtained by curing an ink including such a polyfunctional acrylate has a low flexibility and impact resistance, and therefore, the image thus formed easily cracks on the recording medium or peels off, and the durability of the image is problematic.
Conventionally, in preparing planographic printing plates, so-called PS plates having an oleophilic photosensitive resin layer on a hydrophilic support are used, and the photosensitive resin layer is image-wise exposed to light to form exposed regions having increased or decreased solubility in an alkaline developing solution, and non-image regions are dissolved in the alkaline developing solution and removed. However, along with recent widespread computerization in which image information is electronically processed, stored and output with a computer, there is a need for a new image output method applicable to computerization. In particular, methods of producing a printing plate without use of a developing solution are being studied, and a method in which a planographic printing plate is directly prepared by using an ink composition for ink jet recording has been considered (see, for example, JP-A No. S54-117203). In this method, a printing plate with a desired image, which is preferably hydrophobic, is formed by image-wise ejecting ink onto the surface of a preferably hydrophobic support in an ink jet recording manner and irradiating the ink with actinic radiation to cure the ink. In order to form image regions of a planographic printing plate, it is preferred that ink droplets deposited on the support cure rapidly without bleeding, it is preferred that the cured image regions have excellent strength and strong adhesiveness to the support, and, when the planographic printing plate is being loaded in a printing machine, it is preferred that the image regions conform to the deformation of the support without generation of damage such as cracks. Accordingly, there is currently a need for an ink composition suitable for such an application.